The present invention relates to amorphous metals and, more particularly, to an apparatus and method for slitting an amorphous metal foil.
The invention herein disclosed is based on research sponsored in part by the Electric Power Research Institute, Palo Alto, Calif.
The basic principles upon which the slitting apparatus and method of the invention rely are described in copending U.S. patent application Ser. No. 574,233 filed on the same date as the present application. To the extent necessary for full disclosure of the present invention, the pertinent parts of the copending application are repeated herein.
Amorphous metals are metal alloys in which the usual crystalline structure is not present. A resume of amorphous metal materials and their properties is contained in a paper entitled METALLIC GLASSES: A MAGNETIC ALTERNATIVE, by Donald Raskin and Lance A. Davis, published in IEEE Spectrum, November 1981, the contents of which are herein incorporated by reference. In brief, an amorphous metal is formed by cooling a molten alloy at such a high rate (typically exceeding a million degrees per second) that the usual crystalline structure of the metal does not have time to form. Instead, the metal is frozen into a metastable condition in which the disorder of the molten form is preserved.
Amorphous metals exhibit a number of differences in their properties from the normal crystalline form of the same alloy which make them especially suitable for certain applications. They are harder, more abrasive and more sensitive to mechanical stresses, have higher mechanical strength, flexibility and electrical resistivity than their crystalline forms and some alloys of amorphous metal exhibit the softest magnetic characteristics of any known materials. This latter property is especially desirable for magnetic core materials since the ease with which a magnetic material is magnetized and demagnetized controls the hysteresis losses experienced when the magnetic material is repetitively magnetized first in one direction and then in the other direction as is customarily the case for magnetic core materials in AC machinery.
An AC distribution transformer, for example, has its primary winding permanently connected to the AC line. Thus, the primary winding continuously cycles the transformer core between extremes of magnetic intensity. The repetitive traversing of the hysteresis loop of the transformer core produces hysteresis losses which must be made up by primary power. These hysteresis losses represent an overhead cost of operating the transformer which is independent of load. Even during periods of light or zero secondary loading, power must be fed to the primary to supply the hysteresis losses in the transformer core. Substitution of a suitable amorphous metal for the magnetic steel normally used in transformer cores can reduce this hysteresis overhead by a factor of 10 or more.
Amorphous metals also have characteristics which have heretofore interfered with their use. One of the problems arises because the need for an extremely rapid cooling rate during the casting of a strip of amorphous metal dictates that the amorphous metal strip must be extremely thin. Thicknesses of about 0.076 mm. inches are about the maximum which can be produced, with more typical values on the order of from about 0.025 to about 0.050 mm. inches. Normal transformer core laminations are about ten times thicker. Thus, about ten times as many layers of amorphous metal are required to form a transformer core of the same cross section as are required with the steel laminations of the prior art.
Space factor is important in many magnetic cores. Space factor is defined as the ratio of the actual volume of core material to the physical volume consumed by the core. If the layers making up the core do not lie flat upon each other but instead remain separated by air or other non-magnetic material, the physical volume consumed by the core increases without a corresponding increase in magnetic properties. If burrs or irregularities are present on the edges of the core laminations, the laminations do not lie flat and consequently the space factor is degraded. The thinness dictated by the way amorphous metals are made adds to the problem. For example, burrs which are small enough to be ignored on the edges of conventional core laminations may cause severe degradation in the space factor when ten or more times as many layers are used.
Amorphous materials are so hard and abrasive that it is extremely difficult to cut the as-cast strip into the sizes and shapes needed to form a core. Conventional cutting techniques include, for example, slitting with wheel-type slitting devices, scissors-type cutters and punch-type cutters, all of which rely on sharp cutting edges for clean cuts. The required sharp cutting edges of these devices rapidly degrade due to the hard abrasive material being cut, even when the cutting edges are made of hard material such as, for example, tungsten carbide. Wheel-type slitting devices also suffer from the thinness of the amorphous metal strip. That is, if the amorphous metal strip is, for example, about 0.0015 inches thick, then the cutting wheels must be set for a clearance on the order of 0.005 inches or less. Such tolerances call for the best, and most expensive, attainable tolerances and, even when such tolerances are attained, the cutting must be performed under controlled temperature conditions. When the cutting edges wear, they begin to produce kerfs or burrs which prevent successive layers of a core from laying in complete contact with each other and thus result in degraded space factor.
U.S. Pat. Nos. 4,328,411 and 4,356,377 disclose laser and/or electron beam cutting techniques for forming complex shapes by either melting and cutting completely or partially through an amorphous strip or heating it above its crystallization temperature so that the desired cutting line assumes the brittle crystalline form of the alloy which can thereafter be easily broken to separate the desired shape from the remainder of the strip. Although these techniques avoid the degradation in edge quality resulting from worn cutting edges of cutting tools, they still produce reduced space factor due to edge burrs. In addition, the heating that these techniques produce along the cutting line leaves crystallized alloy with a resulting loss in the very magnetic properties in these areas which use of the amorphous material is intended to provide.
Conventional casting techniques are capable of producing amorphous alloy strips in continuous lengths up to several miles long which are long enough to form the entire core of a transformer or other electrical apparatus. In order to utilize such material, extended lengths of the strip must be slit in a continuous process.
Transformer cores may be formed in a toroidal shape or in the shape of a deformed toroid which approximates a rectangular exterior outline by winding a continuous length of amorphous metal strip about a suitably shaped mandrel. Such cores are subsequently wound with primary and secondary windings to complete the electrical functions of the apparatus. It is desirable to systematically vary the width of the amorphous alloy strip from end to end so that the cross section of a core formed from the strip assumes a shape which conforms to the shape of the winding to be placed on it. Preferred cross-sectional shapes for use in transformer cores include, for example, approximations of circles or regular polygons.